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EQUINE VETERINARY EDUCATION / AE / MARCH 2015


species isolated from horses are Trichophyton equinum, Microsporum equinum, Trichophyton mentagrophytes, Trichophyton bullosum and Trichophyton verrucosum (Scott and Manning 1980; Kane et al. 1982; Scott 1994; Gräser et al. 2000; Chiers et al. 2003; Sitterle et al. 2012). Tack (bridles, halters, saddle blankets) often act as fomites. The lesions usually appear first on the axillary/girth area and may spread over the trunk, rump, neck, head and limbs. Initial lesions may be urticarial in nature progressing to multiple focal sharply demarcated scaling, crusting areas. These may progress to the development of thick crusts, or more generally a diffuse moth-eaten appearance with desquamation and alopecia. Less commonly, deeper structures are infected through the hair follicles causing small foci of inflammation and suppuration. Rarely, dermatophytosis may be limited to the coronary band. Diagnosis is by fungal culture; biopsy is less reliable


(Trichophyton species may cause acantholysis, mimicking pemphigus foliaceus on histopathology) (Scott 1994). Hair is the specimen most commonly collected for the isolation of dermatophytes. Using forceps, hairs should be selected that appear stubbled and broken, especially at the advancing periphery of an active, nonmedicated lesion. In addition, surface keratin may be gathered by forceps or skin scrapings from similar areas and inoculated onto the culture medium. The hair and surface keratin of large animals have


large numbers of saprophytic fungi and bacteria. Hence, it is recommended by some clinicians to cleanse the skin prior to taking samples for culture. This may be done by gently cleansing the area to be sampled with water, and allowing it to air dry, although the author does not routinely do this.


Sabouraud’s dextrose agar has been used traditionally in


veterinary mycology for isolation of fungi; however, other media are available with bacterial and fungal inhibitors, such as dermatophyte test medium (DTM). DTM is essentially Sabouraud’s dextrose agar containing cycloheximide, gentamicin, and chlortetracycline as antifungal and antibacterial agents and to which the pH indicator phenol red has been added. Dermatophytes utilise protein in the medium first, with alkaline metabolites turning the medium red. Most other fungi utilise carbohydrate first, giving off acid metabolites, which do not produce a red colour change. These saprophytic fungi will later use the protein in the medium, resulting in a red colour change. However, this usually occurs only after a prolonged incubation (10–14 days or more). Consequently, DTM cultures should be examined daily. Some Aspergillus species and others cause a red colour change in DTM, so microscopic examination is essential to avoid an erroneous presumptive diagnosis. It has been recommended that one or 2 drops of a sterile injectable B complex vitamin preparation be added to culture plates when culturing horses, as one strain of T. equinum (T. equinum var. equinum) has a unique niacin requirement. Skin scrapings and hair should be inoculated onto Sabouraud’s dextrose agar and/or DTM and incubated at 30°C with 30% humidity. A pan of water in the incubator will usually provide enough humidity. DTM may be incubated for 21 days, but cultures on Sabouraud’s agar should be allowed 30 days to develop. The author has used a product which has DTM on one side and rapid sporulating media (RSM) on the other, with a well of water in the centre (DermDuet)15. It is routinely incubated at


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room temperature. T. verrucosum has been reported not to grow on DTM (Scott and Miller 2011). Topical treatment alone is often curative. While 50%


captan (2 tablespoons of the powder in 1 gallon of water) has been touted in the past, and while certainly safe for tack, its effectiveness has been questioned. Lime sulfur (LimePlus Dip)8 1 cup to 1 gallon of water, or bleach 1:10 with water, are both effective, but messy and odiferous. Miconazole or ketoconazole veterinary shampoos are becoming more widely used, and may be as effective. In Europe and Canada, an enilconazole rinse (Imaveral)16 is highly effective. These topical treatments should probably be done at least every week for 6–8 weeks. All in-contact horses should be treated. Systemic treatment is occasionally needed. Griseofulvin’s


efficacy in horses (as well as an effective dose) has not been thoroughly researched. However, a dosage of 100 mg/kg bwt daily for 7–10 days has been advocated, and has been used with good success on a small number of horses by the author. Griseofulvin is a teratogen, and should not be used in pregnant mares. Alternatively, 20% NaI may be given i.v. (250 ml/500 kg horse every 7 days, 1 or 2 times). This also is contraindicated in pregnant mares as it may cause abortion. While medications such as itaconazole and fluconazole have been used to treat horses with systemic mycotic infections there have not been any studies on their effectiveness in dermatophytosis. However, their safety record in horses for the doses used (2–5 mg/kg bwt q. 12 h) are encouraging (Foley and Legendre 1992; Korenek et al. 1994; Taintor et al. 2004). Annecdotally, terbinafine orally at 5 mg/kg bwt for 10–14 days has been noted to be effective. Higher doses consistent with small animal dosing at 20 mg/kg bwt may lead to adverse reactions in some horses (Williams et al. 2011). Vaccination to T. equinum may reduce the incidence of new infections and protect a high percentage (>80%) of vaccinates from infection. These data are based on results with an inactivated vaccine containing both conidia and mycelial elements (Pier and Zancanella 1993), which is not available in the USA. Ideally, in multiple horse situations, 3 areas/buildings/barns


should be designated: one for affected horses, one for unaffected horses that have been cultured but the results are not yet known, and one for culture-negative horses. Personnel should wash their hands and ideally change their outer garments between these areas. All tack, curry combs and blankets should be washed (1:10 bleach or 3–4% chlorhexidine), put through a dryer/heat cycle, then assigned to one horse. The exact species of the yeast Malassezia growing on


horses’ skin is just beginning to be investigated (Nell et al. 2002). In one study, the Malassezia sp. isolated were identified as M. furfur, M. slooffiae, M. obtusa, M. globosa and M. restricta (Crespo et al. 2002). The author has examined several mares with a Malassezia infection between their mammary glands, which was intensely pruritic. The mares rubbed their tail and ventral abdomen (Fig 5). Physical examination showed a dry, greasy-to-the-touch exudate. Cytology of the exudate showed numerous yeast organisms, which were identified on culture as Malassezia species. Treatment with a topical 2% miconazole/chlorhexidine shampoo was curative. The author is aware of other similar cases. However, healthy nonpruritic mares may also have large numbers of yeasts in the intramammary area (White et al. 2006).


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